In the field of polymeric coatings suitable for the protection of metal and other substrates, acrylate based polymers are often used, the monomeric constituents being carefully selected so as to provide coatings exhibiting both strength and flexibility. Epsilon-caprolactone can be reacted with the hydroxyl or carboxyl groups of acrylic or methacrylic monomers or polymers prepared therefrom, and epsilon-caprolactone itself may undergo polymerization using suitable esterification catalysts such as alkali or alkaline earth metal alkoxides, the polymerization involving opening of the caprolactone ring. Fisk et al., U.S. Pat. No. 4,082,816, teaches methods in which ethylenically unsaturated monomers and epsilon-caprolactone are polymerized, by addition and ring-opening polymerization, respectively, in suitable solvent systems. The caprolactone and ethylenically unsaturated monomers may be polymerized in solution concurrently, the polymerization system including both free radical initiators and esterification catalysts, or the polymerization of the ethylenically unsaturated monomers may be carried out first in solvent solution followed by addition to the solution system of the caprolactone which undergoes condensation polymerization. U.S. Pat. No. 4,720,528 (Etzell, et al.) refers to coating compositions containing epsilon-caprolactone-modified acrylic resins polymerized in solution.
In the addition polymerization of ethylenically unsaturated monomers, temperature plays a large role in determining the polymer resin composition. In general, higher polymerization temperatures provide lower molecular weight polymer species having narrower molecular weight distributions, whereas polymerization at lower temperatures gives rise to higher molecular weight polymers with somewhat broader molecular weight distributions. Molecular weight can be controlled by other means, such as through the use of mercaptan chain transfer agents such as t-dodecyl mercaptan.
In general, the addition polymerization of ethylenically unsaturated monomers, particularly acrylic and methacrylic monomers, normally takes place in a solvent solution. The polymerization temperature generally is limited to the boiling points of the common solvents that are used, e.g., about 150.degree. C., and chain transfer agents or other materials are employed to limit the molecular weight of the resulting polymer. Chain transfer agents such as the mercaptans can have adverse effects upon the ultimate properties of polymeric coatings, however, and the mercaptans are generally known to adversely affect the ability of a coating to withstand weathering.